This application is submitted in response to the Program Announcement "Neurodevelopment and Neuroendocrine signalling in Adolescence: Relevance to Mental Health " (PA-09-008). The primary objective of our research is to understand how rising levels of testosterone during male adolescence affect the axon. The secondary objective is to evaluate the possible impact of such effects on the risk of psychopathology during male adolescence. The focus on the axon, a key element in long-range cell-to- cell communication in the nervous system, is motivated by the known impairment of neural connectivity in disorders emerging during adolescence and young adulthood, including depression and schizophrenia. To reach our goal, the proposed research sets the following four specific aims. (1) To evaluate whether or not the timing &dynamics of sexual maturation during early and mid-adolescence predicts structural properties of white matter during late adolescence. To achieve this aims, we will employ multi-modal magnetic-resonance imaging (MRI) of white matter in a large number (n=500) of 18-year old male adolescents selected from a unique longitudinal birth-cohort established in England in early 1990's, namely the Avon Longitudinal Study of Parents and Children (ALSPAC). The timing &dynamics of sexual maturation will be indexed by the age of puberty onset and changes in testosterone levels throughout adolescence, i.e. using information collected in this cohort over the past 10 years. (2) To evaluate the moderating effects of genes involved in testosterone signalling and/or metabolism on the predicted relationship between testosterone levels and structural &functional propertise of white matter. This aim will be achieved by genotyping the 500 male adolescents for functional polymorphisms in the androgen-receptor (AR) gene and the 5-alpha reductase gene;(3) To evaluate whether experimental manipulations of testosterone levels during puberty affect functional and morphological properties of the axon. This aim will be achieved by studying male rats following real or sham castration, with or without supplementation of testosterone. In this animal model, we will measure two main phenotypes: (a) axonal transport using in vivo Mn++ MR imaging at 7 T;and (b) axonal caliber and myelin thickness assessed ex vivo with electron microscopy;and (4) We will evaluate possible relationship between the timing &dynamics of sexual maturation and the risk of psychopathology and assess whether puberty-related changes in white matter modulate this relationship. This aim will be achieved using the existing longitudinal information about puberty and psychopathology collected in the last 10 years in the ALSPAC participants, and by an assessment of the risk of psychopathology at the time of an MR acquisition, i.e. at the age of 18 years. The above research program integrates population neuroscience (Aims 1, 2 &4) with experimental neuroscience (Aim 3), with the ultimate goal to understand how inter-individual variations in the maturation of white matter during male adolescence might affect mental health. PUBLIC HEALTH RELEVANCE: The white matter of a 20-year old man contains a staggering 176,000 km of myelinated axons. Axons ensure smooth communication throughout the brain in two important ways: by conducting electrical impulses and also by transporting various molecules, organelles and proteins from the cell body to the synapse and back. Given the critical importance of neural connectivity for brain functioning, it is very likely that processes influencing maturation of the axon during childhood and adolescence will impact the child's cognitive development and mental health. The proposed research will combine two approaches: population and experimental neuroscience. Taking advantage of an existing cohort of adolescents, we will investigate the relationship between testosterone trajectories during adolescence and structural properties of white matter. We will also test experimentally the effect of testosterone treatment on structural and functional properties of white matter in experimental animals (rats). Altogether, we believe that the proposed series of population-based and experimental studies will provide an integrative answer to our single question: how does testosterone affect the axon and with what consequences? As such, this work is likely to open up new and uncharted avenues into studies of the neural underpinnings of psychiatric disorders that emerge during male adolescence.